I examine Descartes's theory of cognition, taking as a starting point his account of how misperception is possible. In the Third Meditation Descartes introduces the hypothesis that there are ideas (such as the idea of cold) which seem to be of something real but which in fact represent nothing (if, for example, cold is a privation or absence of heat, rather than the presence of a positive quality). I argue, against Margaret Wilson, that Descartes does not think there are any (...) such ideas and that he introduces the hypothesis only in order to formulate an objection to his argument for the existence of God. I argue further that while he agrees with Arnauld in accepting the Aristotelian account of cognition according to which the very objects in the world that we perceive exist in the soul or its ideas objectively, he still has a satisfactory response to Arnauld's objection that since an idea can represent only what it appears to be of, all error must reside solely in our judgment. I claim that Arnauld's objection that an idea represents what it appears to be of is based on the assumption that an idea appears to be of what exists in it objectively. But Descartes makes room for the possibility of misrepresentation by distinguishing between what exists objectively in an idea and what that idea appears to be of. First, he thinks that it is at least coherent to suppose that an idea lacking objective reality could appear to be of something in virtue of its material reality. Since an idea lacking objective reality would not represent any thing that exists in the world, Descartes concedes that it would not misrepresent any actually existing thing, but it could still appear to be of some thing and in that way misrepresent the way the world is. Second, there is reason to claim that like some of his Aristotelian predecessors Descartes holds that what exists in the soul objectively can appear to be other than it is. This interpretation has the implication that Descartes's theory of ideas, in contrast to sense datum theories, is not driven by the motive of finding some entity which is exactly as it appears to serve as the object of immediate awareness. (shrink)